On Sat, May 17, 2014 at 01:50:52PM +0100, Martin wrote: > On 16/05/14 04:07, Russell Coker wrote: > > https://blogs.oracle.com/bill/entry/ditto_blocks_the_amazing_tape > > > > Probably most of you already know about this, but for those of you who > > haven't > > the above describes ZFS "ditto blocks" which is a good feature we need on > > BTRFS. The briefest summary is that on top of the RAID redundancy there... > [... are additional copies of metadata ...] > > > Is that idea not already implemented in effect in btrfs with the way > that the superblocks are replicated multiple times, ever more times, for > ever more huge storage devices?
Superblocks are the smallest part of the metadata. There's a whole load of metadata that's not in the superblocks that isn't replicated in this way. > The one exception is for SSDs whereby there is the excuse that you > cannot know whether your data is usefully replicated across different > erase blocks on a single device, and SSDs are not 'that big' anyhow. > > > So... Your idea of replicating metadata multiple times in proportion to > assumed 'importance' or 'extent of impact if lost' is an interesting > approach. However, is that appropriate and useful considering the real > world failure mechanisms that are to be guarded against? > > Do you see or measure any real advantage? This. How many copies do you actually need? Are there concrete statistics to show the marginal utility of each additional copy? Hugo. -- === Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk === PGP key: 65E74AC0 from wwwkeys.eu.pgp.net or http://www.carfax.org.uk --- IMPROVE YOUR ORGANISMS!! -- Subject line of spam email ---
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